Variation in serum levels of ethinyl estradiol was studied in 104 healthy women using oral contraceptives. Over 70% of the variation among women persists after the effects of the time interval between pill ingestion and blood drawing and the day of the menstrual cycle when blood was drawn are accounted for. This 70% residual variation is not accountable on the basis of age, height, weight, blood pressure, cigarette smoking, alcohol use, or total lifetime OC use, although slight relationships between EE blood levels and some of these variables are suggested.